Under what conditions can a train enter a track where CTC is in effect?

A train can enter a track where Centralized Traffic Control (CTC) is in effect when a controlled signal displays a proceed indication. This condition ensures that the train operates safely and in accordance with the operational procedures designed to maintain safety and efficiency on the rail network.

Controlled signals are used in CTC systems to convey important information to the train crew, indicating whether it is safe to proceed, stop, or take other necessary actions. The proceed indication from a controlled signal confirms that the track ahead is clear and that the train can continue its journey without risk of encountering another train or obstruction. This adherence to signal indications is critical for avoiding collisions and ensuring the safe movement of trains within the regulated frameworks of CTC operations.

The conditions outlined in the other options do not provide the necessary safety guarantees that a signal indication does. For instance, visual signals alone may not convey the full regulatory information required for safe operations, and restrictions based on time of day or arbitrary waiting periods lack the decisive clarity provided by a controlled signal. Therefore, the correct condition is reliant on the signal's indication of the track's status.